Prestressed concrete members



Oct. 17, 1967 H. K. PRESTON, JR 3,347,005

PRESTRESSED CONCRETE MEMBERS Filed Feb. 9, 1965 I INVENTOR. Howard Kenr Presfon,Jr.

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United States Patent Ofi ice 3,347,005 Patented Oct. 17, 1967 3,347,005 PRESTRESSED CONCRETE MEMBERS Howard Kent Preston, Jr., Morrisville, Pa., assignor to CF&I Steel Corporation, a corporation of Colorado Filed Feb. 9, 1965. Ser. No. 431,263 3 Claims. (Cl. 52-230) ABSTRACT OF THE DISCLOSURE Prestressed concrete members such as girders, beams and the like having at least one tendon strand under tension having trough-shaped spaces extending helically lengthwise of its surface to enhance the mechanical and surface bonding of the concrete to the tendon strand.

In a prestressed concrete member, the concrete on the tensile side of the member is placed under compression before the member is put in service. When such a member is loaded, there are no tensile stresses developed. Since the area where tensile stresses would normally develop is already in compression, there is only a decrease in the compression stress instead of the development of a tensile stress. Prestressed concrete therefore permits the design of crackless structures which require considerably less concrete than corresponding reinforced concrete structures. Most prestressed concrete tendons are composed of high-strength wires either singly or in the form of strands. Tendons are also made from specially designed high-strength steel bars. In this country a high percentage of the tendons used in prestressed concrete are 7- wire uncoated stress relieved prestressed steel strands. At the present time the most popular sizes are 1 inch and /2 inch in diameter.

Difficulties have been encountered in using tendons larger in diameter than /2 inch because of inadequate bonding of the concrete to the strands. It has been the accepted practice to use arrangements of the wires in the strands presenting a relatively smooth surface, such as a seven Wire strand.

This invention is based on concrete members prestressed with tendon strands which are constructed to provide both surface and mechanical bonding. Surface bonding is a function of the area of the strands in contact with the concrete, but the mechanical bonding is achieved in this invention by an arrangement of the exterior or peripheral wires of the strands to form a rib-like exterior having alternate spaces for receiving the concrete, thus forming an interlocking mechanical connection between the strand and the concrete. This combination of surface and mechanical bonding can be achieved advantageously with a thirteen wire strand having seven interior wires and a surrounding layer of six wires, or an eleven wire strand having six interior wires and a surrounding layer of five wires, or a fifteen wire strand having eight interior wires and a surrounding layer of seven wires. It is also possible with a nine wire, seventeen wire, and other combinations. The exterior layer of wires is applied by a helical wind in the same direction as the underlying layer of wires so that each exterior wire lies in a groove between adjacent underlying wires forming alternate longitudinally extending spaces. When the concrete is poured over this strand it enters these spaces forming not only a new kind of mechanical bond but a greater area of surface bond in comparison with a conventional strand having the same number of wires.

In the accompanying drawings FIG. 1 is a side view with one end drawn in perspective of a section of a l3-wire strand;

FIG. 2 is a side view of a concrete beam in process of setting with a tendon strand in position and with the opposite ends of the strand projecting to enable the strand to be placed in tension:

FIG. 3 is a side view of the completed beam;

FIG. 4 is a view in vertical section taken on line 4-4 of FIG. 3 and drawn to an enlarged scale;

FIG. 5 is a fragmentary view similar to FIG. 4, showing a modified tendon strand;

FIG. 6 is a view similar to FIG. 5 showing another modified tendon strand.

Referring now to the drawings, a l3-wire strand 10 is shown in FIG. 1. The interior portion of this strand consists of a central wire 11 and an outer layer 12 of helically wound wires. The central wire 11 is straight with the layer 12 wound around it, the total number of interior wires being seven. On the exterior surface of these interior wires a peripheral layer of wires 13 is wound helically in the same direction as the outer layer 12 of the interior wires. Wires 13 lie in the valleys formed between adjacent pairs of the wires forming outer layer 12 so that each wire 13 lies in contact with each of two wires in the layer 12, thus forming a stable structure. With the wires forming the peripheral layer 13 in position comparatively wide longitudinal spaces 14 after the manner of troughs of generally U-shape are formed between each pair of these wires 13. It will be understood that these spaces extend throughout the entire length of the 13-wire strand 10.

The concrete beam or girder 15 of FIG. 3 is reinforced with at least one of the 13-wire strands 10 which is advantageously located closer to the bottom of the girder than the top as illustrated. In molding the prestressed concrete beam 15 the strand (or strands) 10 is placed in an appropriate mold (not shown), the portions 16 of the strand projecting beyond the ends of the mold as shown in FIG. 2. These end portions are grasped by any suitable tensioning apparatus (also not shown) and the strand is placed under tension as indicated by the arrows in FIG. 2. The concrete is poured in direct contact with strand 10 and allowed to set for the required number of hours. Thereafter the tension producing apparatus is released from the end portions 16 and these end portions are cut off flush with the ends of beam 15 as shown in FIG. 3 to form the completed beam or girder.

During the pouring of the concrete it is caused to flow around the strand 10 and to enter and occupy the longitudinal spaces 14 from end to end of strand 10 and also from end to end of the beam. Consequently after the concrete has set, both surface and mechanical bonds are formed between the strand 10 and the concrete of the beam. The surface or adhesive bond between the concrete and strand 10 is greater than in the case of a smoothly wound cable having the same cross-sectional area as the wires of strand 10 because of the extra surface of the peripheral layer of wires 13 which is exposed to the concrete. In addition a mechanical bond exists between the concrete and strand 10 because of the presence of the six spaces or troughs 14 which extend lengthwise of the beam 15 and strand 10 from end to end thereof. The presence of the mechanical bond and the increased surface bond produces a connection between the reinforcing strand 10 and the concrete beam 15 which enables tendons of larger diameter than /2" to be employed without fear of deterioration in the connection between the tendon and the beam. Indeed to produce a bodily lengthwise movement of one member with respect to the other would be almost impossible since it would necessitate the shearing off of the elongated concrete portions existing in the longitudinal spaces 14 from one end of the beam to the other.

Although it is advantageous to use the 13-wire strand of FIGS. 1 and 4, the prestressed concrete members in the form of beams, girders, fioor panels, etc., may also be reinforced by the use of the strands having six or eight interior wires surrounded by a layer of five or seven The peripheral layer consists of five wires 19 which are helically wound in the same direction as wires 18 and which provide between them five lengthwise spaces 20 for the reception of concrete.

In the modified tendon member shown in FIG. 6 the interior wires comprise a straight central Wire 21 and seven wires 22 helically wound around and in contact with it. The peripheral layer is formed of seven wires 23 which are helically wound in the same direction as the wires 22 which form the outer layer of the interior wires. This arrangement again provides a series of lengthwise recesses or spaces 24 between each pair of the peripheral Wires 23 for the reception of concrete for the purpose of forming a mechanical bond between the tendon and the concrete of the beam in which it is molded.

It will be understood that instead of employing only a single tendon strand for reinforcing a concrete beam or girder, or other member having particular dimensions, two or more of these tendons of the same or different sizes may be employed. Also while it may be advantageous to place these several tendon strands in the same plane, they may be arranged in a different geometrical arrangement if desired.

Because of the greater bondbetween the tendon strand and the concrete which is obtained in accordance with the present invention, concrete beams or girders, or other concrete members of greater dimensions than heretofore can be successfully produced and used in building structures.

I claim:

1. An improved prestressed concrete member having at least one tendon strand under tension, said strand having a plurality of interior wires including an outer layer of helically wound wires, and a peripheral layer of wires helically wound in the same direction as the outer layer of wires, each wire of the peripheral layer lying in a groove between adjacent wires of the outer layer providing spaced helically extending trough shaped spaces between the peripheral wires in which each of said troughshaped spaces is formedby three wires only, the sides of each trough-shapedspace being formed by the surfaces of two adjacent wires of the peripheral layer and the bottom thereof by the surface of a wire of the outer layer thereby providing a longitudinal space having a comparatively wide bottom surface and forming a shallowv U-shaped trough for the reception of the concrete.

2. An improved prestressed concrete rnemberas defined in claim 1 in which the tendon strand and the concrete member are coextensive in length, and wherein the tendon and concrete member are bonded together uniformly throughout their length.

3. An improved prestressed concrete member as defined in claim 1 in which the strand consists of six interior wires having a surrounding layer of seven peripheral wires.

References Cited UNITED STATES PATENTS 811,560 2/1906 Hinchrnan 52733 3,092,956 6/1963 Naysrnith 57-145 3,115,727 12/1963 Middendorf 52-223 7 FOREIGN PATENTS 71,659 1/1953 Netherlands.

FRANK L. ABBOTT, Primary Examiner,

J. L. RIDGILL, Assistant Examiner 

1. AN IMPROVED PRESTRESSED CONCRETE MEMBER HAVING AT LEAST ONE TENDON STRAND UNDER TENSION, SAID STRAND HAVING A PLURALITY OF INTERIOR WIRES INCLUDING AN OUTER LAYER OF HELICALLY WOUND WIRES, AND A PERIPHERAL LAYER OF WIRES HELICALLY WOUND IN THE SAME DIRECTION AS THE OUTER LAYER OF WIRES, EACH WIRE OF THE PERIPHERAL LAYER LYING IN A GROOVE BETWEEN ADJACENT WIRES OF THE OUTER LAYER PROVIDING SPACED HELICALLY EXTENDING TROUGH SHAPED SPACES BETWEEN THE PERIPHERAL WIRES IN WHICH EACH OF SAID TROUGHSHAPED SPACES IN FORMED BY THREE WIRES ONLY, THE SIDES OF EACH TROUGH-SHAPED SPACE BEING FORMED BY THE SURFACES OF TWO ADJACENT WIRES OF THE PERIPHERAL LAYER AND THE BOTTOM THEREOF BY THE SURFACE OF A WIRE OF THE OUTER LAYER THEREBY PROVIDING A LONGITUDINALLY SPACE HAVING A COMPARATIVELY WIDE BOTTOM SURFACE AND FORMING A SHALLOW U-SHAPED TROUGH FOR THE RECEPTION OF THE CONCRETE. 